1. Field of Invention
This invention relates to modeling of operation of complex large scale integration (LSI) or very large scale integration (VLSI) digital devices for use in development and testing of digital systems. More specifically, the invention relates to logic simulation and testing of complex digital circuitry and systems including those capable of executing instructions under program control in which digital performance characteristics of static or dynamic LSI or VLSI devices must also be accurately simulated.
A logic-simulation model of a device is a diagnostic tool which accurately mimics digital logic and timing behavior of a device in normal operation. The purpose of such a model is to verify both logic and timing of an operational digital system containing the device. In a logic-simulation model, internal operation and internal structure need not be similar to that of the actual device being simulated. The only prerequisite is that the simulated operation and simulated timing as externally observed be identical to that of the actual device being modeled.
2. Description of the Prior Art
Traditionally, logic-simulation models have been implemented with software. Software logic-simulation models have been of two types, namely structural models and behavioral models. A structural model mimics actual internal logical structure of a device from which observable functional behavior follows. A behavioral model merely mimics external logical and timing behavior.
Software models of complex devices have numerous disadvantages. First, they are relatively costly and time consuming to develop. To design an accurate model, specifications of the device must be gathered and thoroughly understood. This has been a serious limitation because manufacturers of devices are generally reluctant to disclose such details. Moreover, the specifications required for modeling a device are typically much more detailed than those relevant to a typical user of the device.
Second, software simulation models are characteristically slow because of the amount of computation required to simulate device functions. Typically, the amount of computation required to simulate external components in a full system being modeled is negligible compared with the amount of computation required for the complex device itself. In fact, software logic simulation models are frequently too slow to be of practical utility.
Heretofore, there have been few tools available to simulate the operation of a dynamic digital device in real-time using a physical device. Some diagnostic tools are known, as for example an In-Circuit Emulator (ICE) from Intel Corporation of Santa Clara, Calif. The In-Circuit Emulator provides means for cycling microprocessor devices and for stopping at well-defined points during operation, such as during a device idle state. Consequently, there is no capability of or suggestion for resetting the device during normal operation of the system.
As complex devices become more dense, the problems of simulation, including development cost, model accuracy, and the requirements to simulate at high speed can be expected to become acute. What is therefore needed is a diagnostic tool for simulating operation of complex digital devices, in particular dynamic digital devices, for use in developing and testing larger systems which requires only minimal relevant information and which enables a system under development or test to be simulated using a known good device.